This invention relates to a pressure sensitive adhesive (PSA) tape of high tensile strength and low elongation in a tape width less than 2.0 inches.
Pressure sensitive adhesive tapes with which the present invention is concerned are widely used as tear tapes, but in addition it may be possible to use tapes of the present invention as pressure sensitive strapping, as reinforcement for corrugated products and for similar purposes.
Tear tapes are widely used in packaging to tear a packaging material along a tear line defined by the tear tape. This is used in filmic packaging materials but also can be used with paper or board materials such as for soap boxes or courier envelopes. The tear tape thus forms a very small part of the packaging material but provides a function which is very important for customer satisfaction. It is of course desirable therefore to reduce the amount of material and cost involved in the tear tape construction.
Tear tapes conventionally comprise a strip of a suitable plastics material and generally polypropylene is chosen as this is relatively inexpensive and yet provides the required strength characteristics. The tear tape can have a width generally in the range 1/16th to 1/4 inch. The width is generally selected on the basis of minimum cost.
It is generally necessary to bond the tear tape to the packaging material in order to hold the tear tape in place during the manufacturing process and to locate the tear tape in place while it is tearing the packaging film to provide a clean straight tear line.
Some packaging processes include the application of adhesive to the tear tape on the packaging line so that an adhesive applicator is provided as part of the machine and applies the adhesive to the tear tape immediately prior to its application to the packaging film.
More recently tear tapes have been supplied in package form carrying a pressure sensitive adhesive on one surface. Such tapes are formed by slitting from a web of the tape material on which is already applied the pressure sensitive adhesive. This type of tape provides some problems in that the adhesive material extends to the side edges of the tape and therefore engages guides to cause transfer of adhesive material to the guides and can spread from the side edge of the tape onto other parts of the packaging material which interfere with the proper operation of the structure. A tape of this type is shown in U.S. Pat. No. 4,844,962 (May et al assigned to Payne Packaging Limited).
In prior U.S. Pat. No. 4,887,714 (O'Connor assigned to KT Industries Inc.) is shown a modified form of tear tape in which the pressure sensitive adhesive is applied as a narrow bead to the tear tape core layer after the tear tape has been slit from a web of the film. This process has been commercially successful and operates satisfactorily on the packaging machine.
Other types of pressure sensitive adhesive tape are also widely used. A tape as defined herein is a core layer of an extruded polymer material having a pressure sensitive adhesive on one side thereof, low elongation to break and a width less than 2.0 inches. Low elongation to break is defined herein as being generally of the order of 10 to 20% and certainly less than 40%. Tapes of this type can be used for various purposes including tear tapes as mentioned above which conventionally have a width in the range 1/4 down to 1/16 inch. Tabbing tapes often have a width in the range 1.0 to 2.0 inch. Reinforcement tapes often have a width in the range 0.5 to 1.0 inches. In all cases the above low elongation is required for the end use stated both in the finished product and also during processing of the tape to form the finished product during packaging of the tape, during unwinding of the package and also during application of the tape to the required substrate.
An extruded material is disclosed in Canadian Patent 977,629 (Plicoflex Houston Inc.)in which a strip of material is extruded onto a drum and cooled following which it is then transferred to a second drum and an adhesive material is applied to one surface. Downstream of the drum the tape is drawn by 25% to 50% which creates a material which is heat shrinkable and of high elongation. The tape of Plicoflex is often referred to in the specification as a film and it is certainly not clear whether the material as extruded is intended to be extruded in the width of the finished tape or whether slitting is intended. In any event the finished tape is intended to have a width greater than 2.0 inches in view of the stated end use of the tape. The material as extruded must be relatively wide (much greater than 2.0 inches) in view of the technique disclosed for applying the adhesive. The patent also refers on a number of occasions to "trimming" the film. Such a tape material as disclosed in Plicoflex is entirely unsuitable for the tape structure with which the present invention is concerned, that is the high strength, low elongation tapes as defined above.
A further coextruded pressure sensitive adhesive material is disclosed in U.S. Pat. No. 4,379,806 (Johnson & Johnson) in which a base polymer film is extruded with an adhesive material with the coextrusion causing a bonding effect between the layers. The patent interchanges use of the words "tape" and "film" but it is almost certainly clear that this material is indeed extruded in web width since the patent refers to trimming of the edges of the material and slitting the material into tape width using conventional techniques.
Generally, tear tapes are manufactured by slitting an extruded web of polypropylene or other suitable material which is oriented either monoaxially or biaxially by conventional processing. This web is then slit into a plurality of longitudinally continuous side-by side tapes which can be coated with adhesive in a process subsequent to the slitting as in the O'Connor patent set out above or can be coated in web width with the adhesive prior to slitting as in the May patent above.
There is an ongoing requirement to yet further reduce the costs of manufacture of PSA tape and to manufacture a PSA tape with minimum materials and minimum waste of materials in an efficient and cost effective manner.